1. Field of the Invention
This invention relates to the gasification of coal, and more particularly it relates to the preparation of a high-solids concentrated low rank cool slurry for a coal gasification process.
The gasification of solid fuels, such as low rank fuels, e.g., coal, is well known. Several methods have been proposed for such a procedure. In one method the solid fuel is ground to a fine powder and fed to the gas generator as a suspension in a vaporous medium, e.g., steam or in a gaseous medium such as a free oxygen-containing gas. However, this method is unsatisfactory as it is difficult to control the amount and rate of solid fuel fed to the gas generator. In addition, if the solid fuel is suspended in a free oxygen-containing gas, care must be taken to maintain the velocity of the suspension above the rate of flame propagation to avoid a backflash which to say the least, is indesirable.
One phase of our invention is concerned with the production of a slurry of solid fuel in water which slurry is suitable for feed to a generator for the gasification of the solid fuel by partial oxidation. One problem presented in such a gasification procedure is that the rate of feed of the solid fuel should be constant. When the solid fuel is suspended in a gas such as steam or oxygen, it is difficult to feed the fuel at a constant rate in that on occasion a slug of fuel may enter the gasifier resulting in a surplus of fuel. This means that some fuel will pass through the gasifier unconverted. At other times there may be a temporary fuel deficiency resulting in a surplus of oxygen with the resulting conversion of a portion of the solid fuel to CO.sub.2 rather than the desired CO. One satisfactory commercial method of controlling the rate and composition of the feed is to introduce the feed into the gasifier as a slurry in water. However, because of the short residence time of the solid fuel in the gasifier it must necessarily be finely ground so that in such a short space of time it may be substantially completely gasified. Frequently, a slurry of low grade solid fuel, e.g. sub-bituminous coal or lignite, to be pumpable, contains from about 35 to 50% water. Although such a slurry is pumpable, it is unsatisfactory as the excessive amount of water has a detrimental effect on the thermal efficiency of the furnace. It is therefore important for the satisfactory operation of the gasifier that a water-solid fuel slurry used as feed have a high solids content and yet be pumpable.
The situation is aggravated when the solid carbonaceous fuel is of low quality such as sub-bituminous coal, lignite or peat.
Millions of tons of low rank fuels such as sub-bituminous coal and lignite exist in this country and although many of the deposits are readily mined, they are not used extensively as fuels, because for the most part, they are located at substantial distance from the point of ultimate use and, in addition, they have several characteristics which make them less attractive as fuels. For example, although generally they have a relatively low sulfur content, these low rank fuels still contain too much sulfur to permit their use as a fuel and still meet current regulations with respect to SO.sub.2 emissions. In addition, to take these coals economically attractive, means must be found for separating the components of the coal having little or no heating value from the components having a high heating value. Thus inorganic mineral matter, water and carbon dioxide ar desirably removed from such fuels to produce a fuel having a higher BTU/lb value and thereby produce a fuel which is more economic to transport either by rail or pipeline.
Therefore, it is an object of this invention to remove as much moisture as conveniently practical from the solid prior to its transportation or use. Also, it is an object of this invention to dehydrate low ranking solid fuel and inhibit rehydration of the surface with straight chain hydrocarbons for preparing slurries with higher BTU contents and higher solid concentration levels.
2. Disclosure Statement
U.S. Pat. No. 2,610,115 discloses both a method and an apparatus for drying lignite and similar high moisture content lignitic fuels to produce a relatively moisture-free, stable and storable fuel.
U.S. Pat. No. 3,996,026 discloses a process for feeding a high solids content solid fuel-water slurry to a gasification process where the water content is between 35 and 55 wt. %.
U.S. Pat. No. 4,047,898 discloses a process for the hydrothermal treatment, i.e., upgrading, of a low rank solid fuel in the presence of hydrogen and heating a particulate mixture of such solid fuel in an inert atmosphere to a temperature between about 300.degree. and 700.degree. F.
U.S. Pat. No. 4,057,399 discloses a process for dewatering carbonaceous materials such as coal by a treatment with hydrogen at elevated temperatures.
U.S. Pat. No. 4,104,035 discloses a method of preparing a solid fuel water slurry by heating to a temperature above 300.degree. F. under sufficient pressure to maintain the water in a liquid phase, and adding a surface active agent to form a mixture having a water content between 40 and 50 wt. %.
U.S. Pat. No. 4,166,802 discloses a process for the gasification of low quality solid fuel where a sufficient amount of moisture is removed to provide a high solids content fuel for such gasification process.
U.S. Pat. No. 4,212,112 discloses a method of drying solid carbonaceous materials such as coal by treatment with an azeotrope, i.e., water and benzene at a temperature of 156.degree.-320.degree. F. for a period of about 10 minutes to about 4.0 hours.
U.S. Pat. No. 4,223,449 discloses a process for dewatering hydrocarbonaceous solid fuels with a solvent such as benzene or toluene at elevated temperatures, i.e., 200.degree.-600.degree. F., and a pressure of about 1 to 100 atmospheres.
U.S. Pat. No. 4,304,572 discloses a process for improving the pumpability of a high solids content water slurry of a solid fuel (e.g., sub-bituminous coal) by forming a slurry containing NH.sub.4 OH and a surface-active agent comprising a salt of an organic sulfonic acid.